1. Field of the Invention
The present invention relates to a circuit structure of a pixel region of an active matrix type display device using a thin film transistor (TFT), and particularly to a structure of an auxiliary capacitance.
2. Description of the Related Art
In recent years, the demand for an active matrix type liquid crystal display device has been increasing, and a technique for manufacturing a thin film transistor (TFT) using a semiconductor thin film (several tens to hundreds nm in thickness) formed on a substrate having an insulating surface has been rapidly developed.
In the active matrix type liquid crystal display device, a thin film transistor is disposed for each of several tens to millions of pixels arranged in matrix, and an opposite electrode is provided on an opposite substrate for each of pixel electrodes through an intervening liquid crystal so that a kind of capacitor is formed. A voltage applied to each of the pixel electrodes is controlled by the switching function of the TFT and an electric charge to the capacitor is controlled, so that the liquid crystal is driven and the amount of transmitting light is controlled, whereby a picture is displayed.
Since the holding capacitance of the capacitor is gradually decreased by an electric current leak, the amount of transmitting light is changed and deterioration in the contrast of picture display has been caused.
Then, a capacitive wiring line is conventionally provided so that a capacitor (auxiliary capacitance) different from the capacitor with the liquid crystal as a dielectric is disposed in series. This auxiliary capacitance serves to compensate an electric charge lost by the current leak from the capacitor with the liquid crystal as the dielectric.
As typical structures of this auxiliary capacitance, structures using, as a dielectric, (1) a gate insulating film, (2) an interlayer insulating film, and (3) a passivation insulating film are enumerated. Especially, since the structure of (1) has large capacitance per unit, it is often used.
However, conventionally, if an auxiliary capacitance using a capacitive wiring line is formed to secure sufficient capacitance, an aperture ratio is lowered. Particularly, in such a small (not larger than 3 inches) and highly fine panel as is used for a projector display device, since a pixel area is small, lowering of the aperture ratio due to the occupied area of the capacitive wiring line has been a problem.
As a phenomenon intrinsic to a display device using a liquid crystal, a light leak occurs from slightly poor orientation of the liquid crystal. Although the liquid crystal is arranged with orientation of some regularity between a pixel electrode and an opposite electrode, there may occur a case that the orientation is disturbed due to roughness of the surface of the electrode, poor rubbing, or the like. The light leak generated in the portion between a region where the orientation of the liquid crystal is disturbed and a region where the orientation of the liquid crystal is normal is called disclination. This disclination lowers display contrast.
As means for preventing this disclination, means for concealing the portion of occurrence of disclination by a black mask (BM) has been adopted. However, since this disclination is generated by a slightly poor orientation of the liquid crystal, it has been difficult to predict the portion of occurrence. Besides, there has been a problem that the aperture ratio is lowered by providing the black mask for concealing the light leak.